We have developed a DBA/2 murine model in which a methylcholanthrene induced syngeneic lymphoma L5178Y can be suppressed to a dormant state for as long as 12 months. During this tumor dormant state, mice are clinically normal, and L5178Y cells can be readily isolated in the peritoneal cavity by in vitro culture. Thus, L5178Y cells grow out rapidly in vitro, but in vivo both escape lysis by cytolytic T lymphocytes, are restrained from rapid outgrowth, and persist for prolonged periods of time with the host remaining clinically normal. Eventually, however, some event occurs which permits the tumor cells to grow out to an overt ascitic tumor. We propose to continue an extensive analysis of this model. We will identify the characteristics of the tumor cell population and of the murine host which permit the establishment and maintenance of the tumor dormant state. This will be done by an analysis and comparison of cells used to initiate dormancy with those isolated during the dormant state and those emerging after its termination. The interaction of host cells with themselves and with tumor cells during dormancy will receive special emphasis. We will attempt to find ways to either destroy all dormant tumor cells or prolong the tumor dormant state to the lifespan of normal mice. The murine tumor dormant model is reproducible and manipulable, and the biological principles elucidated in its analysis should be applicable to the understanding and control of tumor dormancy in man.